1. Field of the Invention
The present invention relates to a die set for press forming a metal sheet such as a sheet steel or an aluminum sheet mainly applicable to an automobile body, and a press forming method using such a die set. More particularly, it relates to a die set for press forming and a press forming method, each capable of minimizing the occurrence of dimensional accuracy defect of a formed product caused by elastic recovery after release from the die set in press forming.
2. Description of Related Art
In the motor vehicle related industry, high strength materials tend to be increasingly used because of a growing demand for an improvement of the crash safety of an automobile body and environmental protection (improvement of fuel economy due to a reduction in weight). A large number of parts of an automobile body are generally manufactured by press forming metal sheets. However, when these parts are formed by press forming, the shape (dimension) of the resulting formed product varies from the designed value due to the elastic recovery behavior after release from the die set (taking out from the die set after forming), which may cause deficiencies at the time of assembling of parts or at the time of bonding (often, bonding by spot welding). Such deficiencies are generically referred to as dimensional accuracy defects. As such dimensional accuracy defects, various ones such as wall warp and angle change are known (see, e.g., “Press Forming Relative Difficulty Handbook” second edition (1997), pages 175 to 196, THE NIKKAN KOGYO SHIMBUN, LTD.).
In recent years, with the growing opportunities to use a sheet steel having higher strength, and an aluminum sheet having a smaller weight than that of a sheet steel, but having a low Young's modulus for an automobile body from the viewpoints of a reduction in weight and the stability of the automobile body, the foregoing dimensional accuracy defect has become a noticeable problem.
FIG. 1 is an explanatory diagram showing an outside shape example of a hat channel member as an automotive part. Such a hat channel member is mainly formed by a draw forming method [FIG. 2A] and a bend forming method [FIG. 2B].
One example of the dimensional accuracy defect when a hat channel member was formed using a die set (a die set for draw forming) shown in FIG. 2A will be described by reference to FIGS. 3A and 3B. FIGS. 3A and 3B each show a product cross sectional shape when a 980 MPa class cold rolled sheet steel (thickness t: 1.2 mm) has been formed. It is indicated that the shape after press forming [FIG. 3B] largely deviates from the objective dimensions. Namely, when the designed (objective) shape (axially perpendicular cross sectional shape) of the hat channel member is assumed to be the one shown in FIG. 3A, the flange surface to be bonded to other parts by spot welding or the like, and to be required to have a close dimensional accuracy springs up by as much as 48° (this spring up angle will be referred to as a “flange spring angle θ”, below). This is considered to be caused by the effects of both the angle change defect at a punch shoulder and the wall warp defect at a sidewall portion [FIG. 3B]
It is explained that the occurrence mechanism of the wall warp arising at the sidewall portion of the hat channel member is due to the following mechanisms (1) to (3) (see the aforesaid document).
(1) The material (metal sheet) undergoes bending deformation when passing through the die radius portion;
(2) When this portion flows from the die radius portion to the sidewall portion, it undergoes bending-back deformation to be stretched in a straight line, and at this step, a difference between stresses of opposite signs occurs along the sheet thickness direction at the sidewall portion, so that the bending moment due to the difference in stress inherently exists; and
(3) When the resulting formed product is released from the die set after forming, elastic recovery is generated so as to release the bending moment, and as a result, warp occurs.
As techniques for reducing such a wall warp phenomenon, various proposals have been made heretofore. As one of such techniques, there is known a method utilizing the reverse bending in a die gap (between a die and a punch (see, the Non-patent Document 1)). The mechanism for wall warp reduction in the case where this method is applied is described as follows.
First, as with general forming, when a material passes through the die radius portion, it undergoes bending deformation. However, when this portion flows from the die radius portion to the sidewall portion, there may occur a phenomenon that the material does not completely wind around the die radius portion according to setting of the size of the die radius and the clearance (the gap between the punch and the die). This phenomenon is generally referred to as overrun. The material which has flown to the sidewall portion due to this phenomenon undergoes bending in a reverse direction to the foregoing bending direction (generally referred to as reverse bending).
Then, when the material is released from the die set after forming, elastic recovery occurs so as to release the bending moment as with general forming. However, the elastic recovery at this step acts in a direction to cancel out the foregoing resultant reverse bending. For this reason, when the curvature of reverse bending and the curvature generated due to elastic recovery become equal to each other, these cancel out each other. As a result, it becomes possible to set the curvature of the sidewall portion (=wall warp) to 0.
As a method for controlling the wall warp by utilizing the overrun, there is conventionally known a method in which the die radius and the clearance are properly controlled. However, with such a technique, the die radius and the clearance are required to be controlled accurately in order to entirely eliminate the wall warp. Particularly, the technique will not exert its effects unless the die radius (rd) is controlled at rd/t (t: thickness)=about 1.5 (see the aforesaid document) The thickness t of the sheet steel generally used for automotive structural parts is about 1 mm. Thus, in order for the technique to effectively exert its effects, it is essential that the die radius (rd) is set at about 1.5 mm.
However, when the die radius (rd) is reduced, unfavorably, the risk of the occurrence of cracking during forming increases so much, and tools become more likely to wear, which necessitates the maintenance of the tools to be frequently performed. These problems can also be said to be destabilizing factors in actual production. For this reason, the foregoing method is unfavorably less applicable to mass production.